The present disclosure relates to a manufacturing method of a laminated structure, a laminated structure and an electronic device, and is preferably applied to, for example, a transparent conductive film used in a display and a touch panel as well as various electronic devices each including a transparent conductive film.
Graphene, which includes a single layer of carbon atoms of graphite, is expected as a transparent conductive material or a wiring material because of its high conductivity. Especially, graphene synthesized by a thermal CVD method attracts attention in that a film can be formed in a large area while the number of layers can be controlled.
In the synthesis method of graphene by the thermal CVD method, graphene is formed on a metal catalyst substrate, typically on a copper foil. Accordingly, the formed graphene needs to be transferred from the metal catalyst substrate onto a desired substrate.
As a transfer method of graphene known to the inventors, the transfer method using polymethyl methacrylate (PMMA), the transfer method using a thermal release tape, and the like have been reported (for example, see Non-Patent Literature 1).
Also, as another transfer method of graphene known to the inventors, there has been proposed the method of forming a graphene sheet on a carbonization catalyst film, forming a binder layer to this graphene sheet, bonding a substrate to the binder layer, and dipping these in an acid solution to remove the carbonization catalyst film (see Patent Literature 1).
Incidentally, as represented by the case where a flexible print substrate (FPC) is manufactured, when a metal foil and a film substrate are bonded together by a roll-to-roll process, an adhesive including a thermosetting resin is usually used. This is because while the use of an adhesive including a UV curable resin causes adhesion to a metal foil to become extraordinarily low and bonding itself to be difficult, the adhesive including a thermosetting resin enables a metal foil and a flexible film substrate to be tightly bonded together.
It is considered that the reasons why the adhesive including a thermosetting resin can achieve high adhesive strength even to the metal foil is that the hardness of the thermosetting resin is not considerably high, and that a curing process is performed after bonding and winding-up.
On the contrary, when the adhesive including an UV curable resin is used, a curing process with UV exposure is performed immediately after bonding, and winding-up is performed after the curing. In this case, the hardness of the adhesive layer rapidly increases in a state where the laminated structure of metal foil/adhesive layer/film substrate is bent and applied with tension. Therefore, the metal foil and the film substrate each having a different hardness cannot be maintained in a state of being bonded together, thereby causing peeling.
On the other hand, when the adhesive including a thermosetting resin is used, the adhesive is flexible until being wound up. Therefore, even when bent and applied with tension, the adhesive layer deforms to relax its stress, and furthermore strong adhesiveness still remains in the adhesive layer during wound-up. Accordingly, peeling is not likely to be caused. In this manner, when the metal foil and the flexible film substrate are bonded to each other by a roll-to-roll process, it is considered that the adhesive including a thermosetting resin is suitable.
Here, in order to manufacture a transparent conductive film including graphene in a large area and with high throughput, a graphene-added metal foil synthesized into a roll shape needs to be bonded to a transparent resin film and the like by a roll-to-roll process. When the graphene-added metal foil and the transparent resin film are bonded to each other per piece, a force in a bending direction or the like is not applied to the film after bonding. Therefore, when the both are only slightly bonded, a transparent resin film can be manufactured by removing the metal foil by etching or the like.
In a case where the transparent conductive film including graphene is manufactured by a roll-to-roll process as described above, it is conceivable that when bonding the graphene-added metal foil to the transparent conductive film or the like, the adhesive including a thermosetting resin is used as in the case of manufacturing a flexible print substrate. However, the thermosetting resin has problems in that (1) the curing requires warming for an extended period, and (2) since a solvent drying process is generally required, an extensive apparatus is required. To solve these problems, it is desirable that the bonding be performed using the adhesive including a UV curable resin.